Providing optimal imaging when using superconducting magnets requires a very uniform magnetic field. Unfortunately, presently it is near impossible to provide a perfectly uniform magnetic field so as to provide a homogeneous field over a sample volume. In fact, one of the key design and operational issues for superconducting magnets is the magnet's spatial field homogeneity. As an example, for a high-resolution Nuclear Magnetic Resonance (NMR) superconducting magnet, its field must be uniform with errors limited to ˜0.01 ppm over, typically, a spherical volume of diameter of 10 mm, within which a sample is placed. In addition, for a whole-body Magnetic Resonance Imaging (MRI) magnet, this spherical volume of diameter can be 20-30 cm.
Field shimming is an essential process used in, for example, superconducting NMR and superconducting MRI magnets, to create a spatially homogeneous field over the sample volume. Present field shimming generally relies on fields generated by two types of coils, superconducting and copper, and by ferromagnetic (steel) material. A superconducting NMR magnet is generally equipped with its own set of superconducting shim coils, currently wound with Niobium-Titanium (NbTi), which is a low-temperature superconductor (LTS). Due to their field limitations (<12 Tesla (T) even at 1.8 Kelvin (K)) and radial build typically of 15-30 mm, NbTi shims are located in an annular space outside of magnetic assemblies, specifically, radially furthest away from a magnet center where the sample is placed.
FIG. 1 is a schematic diagram illustrating an exemplary prior art NMR magnet assembly 2. As shown by FIG. 1, the assembly 2 contains a superconducting NMR magnet 10, corrective coils 20, and conventional exterior superconducting shims 30. As previously mentioned, the conventional exterior superconducting shims are typically NbTi shims. A sample, to be imaged by the assembly 2, is placed within a bore 50, where the bore 50 is located within the NMR magnet 10. The location within the NMR magnet 10 is typically referred to as a premium high-field region 60 of the superconducting magnet. The sample is located within a detection region 40 of the assembly 2, which is typically a 10-mm Diameter Spherical Volume (DSV) established for the sample.
Conventional NbTi shim sets 30 are placed outside of the main magnet, because: 1) an annular space within the main magnet has always been considered better utilized to generate the main field, rather than to improve the efficiency of a shim set; and 2) NbTi shim sets cannot generally be operated in a higher-field region of the main magnet, which is located closer to the center, within the premium high-field region of the superconducting magnet.
Unfortunately, there are inherent technical disadvantages for a shim placed outside of the main magnet and the correction coils. At a great distance the shim must work much harder (i.e., more ampere turns) to generate a required shimming field. One prominent source of field errors is a screening-current field (SCF), also referred to as a diamagnetic field, generated by each coil in a superconductor magnet assembly. The diamagnetic fields generated by the coils create diamagnetic walls that are proportional to the superconductor size and critical current density. The shim field is not only attenuated, but also distorted when it penetrates through these diamagnetic walls to reach the center of the superconducting magnet. In addition, the field attenuation is asymmetric in the axial (z) direction.
Therefore, there is a need to provide a more spatially homogeneous field over a sample volume in the superconducting magnet.